1. Field of Invention
This invention generally relates to a method and apparatus for detecting a surface level of a material in a container. Specifically, the invention relates to illuminating a portion of a surface and detecting a position of the illuminated surface portion to determine and/or control the surface level.
2. Description of Related Art
The Czochralski method is widely used in the semiconductor industry and involves melting a semiconductor material such as silicon in a crucible, dipping a seed crystal into the melt, and withdrawing the seed crystal from the melt. As the seed crystal is slowly withdrawn from the melt, material in the melt crystallizes and forms a larger monocrystalline ingot. The diameter and length of the monocrystalline ingot that is formed is controlled based on various factors, including the pull rate of the crystal from the melt, and the melt surface level.
Various methods and apparatus exist for controlling the melt level in a Czochralski-type crystal growing device, as discussed in U.S. Pat. No. 3,882,319 to Clement et al., U.S. Pat. No. 4,350,557 to Scholl et al., and U.S. Pat. No. 4,915,775 to Katsuoka et al.
However, the methods and apparatus disclosed in these patents involve complex devices, or require an accurate estimate of the amount of semiconductor material that is withdrawn from the melt crucible to form the crystal ingot.